Final answer:
Teenagers embracing counter-cultural ideas were the group that most resonated with Henry David Thoreau's speeches and writings, as his transcendentalist philosophy and emphasis on nature and individualism aligned with their values during the 1960s to 1980s.
Step-by-step explanation:
The age demographic that most responded to Henry David Thoreau's speeches and writings was B. Teenagers embracing counter-cultural ideas. During the tumultuous times of the 1960s through the 1980s, young people were at the forefront of societal change, seeking autonomy and authenticity. They resonated with Thoreau's transcendentalist views, which included a deep reverence for nature and a staunch belief in individualism and self-reliance. Thoreau's experiences, as detailed in his work Walden; Or, Life in the Woods, epitomized a simpler, deliberate life that many young people yearned for in the face of industrialization and societal expectations.
In the context of the growing environmental movement, it was Thoreau's connection to nature and his early ecocritical voice that influenced the youth. Thoreau, alongside Ralph Waldo Emerson, was a precursor to modern environmentalism and influenced the creation of conservation spaces. This notion of returning to nature for inspiration and living a life of simplicity and deliberate choice resonated with the young generation, particularly those involved in the counter-culture who desired peace, rejected societal norms, and worked towards social reforms.